1. Field of the Invention
This invention relates to a light quantity control device adapted for use in a beam recording apparatus or the like.
2. Related Background Art
In a known image recording apparatus, such as a laser beam printer, a semiconductor laser which emits a laser beam is used as a practical light source.
The semiconductor laser has an unique light quantity characteristic (I-l characteristic) between a laser driving current "I" and a emitted light quantity "l" as shown in FIG. 13. In FIG. 13, the vertical axis denotes the light quantity "l", and the horizontal axis denotes the laser driving current "I". The semiconductor laser does not emit the laser beam until the laser driving current "I" reaches a threshold value "I.sub.th ", but emits the laser beam when the laser driving current "I" exceeds the threshold value "I.sub.th " (such region is referred to generically as tHE "LED emitting region"). In the laser beam emitting region, the light quantity "l" has a certain slope ".varies." relative to the laser driving current "I".
In the known image recording apparatus, the laser is set to a non-emitting state (i.e., the laser output is set to zero) at first before initiating the printing of each page. The laser driving current "I.sub.T " is determined in such a way that a desirable prescribed light quantity "l.sub.T " can be obtained by monitoring the laser output and by controlling the laser driving current. The image recording apparatus holds the prescribed light quantity "l.sub.T " constant by performing constant-current driving of the laser driving current "I.sub.T " corresponding to the prescribed light quantity "l.sub.T."
However the I-l characteristic of the semiconductor laser, which has an initial form "A" as shown in FIG. 13, may vary to a form "B" or "C" due to variation in the chip temperature. Accordingly, in the laser beam printer, a constant-current driving circuit executes light quantity control (APC or auto power control) by clearing the laser output before initiating the printing of each page so as always to perform each printing operation with the desirable prescribed light quantity "l.sub.T ".
A method for deciding the current value of the constant-current driving circuit is proposed in U.S. Pat. No. 4,890,288, which method controls the current value by utilizing an IC (D/A converter) for converting a digital value into an analog value.
However, the conventional light quantity control device, which can control the light quantity characteristic to the driving current of the semiconductor laser by utilizing the D/A convertor, suffers from the problem that the bit number of the D/A converter increases in the case where a precise light quantity control is required. That is, the conventional control device takes a lot of time to complete the light quantity control because of the large number of up-counting steps for controlling the drive current increase in the case where the counting operation is initiated from a clear state.